Air-operated scissors-type jack

ABSTRACT

A scissors-type jack for a vehicle lift comprises a base, a scissors linkage above the base and a platform on the scissors linkage adapted to be raised to various elevations by upward extension of the linkage from the base. The linkage is adapted to be so extended by air cylinder actuated follower rollers which have a wedging action between the links of the scissors linkage, the air cylinder being movable with the scissors pivot. An auxiliary locking mechanism is provided for locking the follower rollers in various platform-elevating positions thereof to prevent collapse of the jack under load in the event of failure of the air cylinder to hold the follower rollers in platformelevating position.

United States Patent [72] Inventors Carl B. Mueller; FOREIGN PATENTS lemme wegmanm 1,175,852 8/1964 Germany 254/122 George Weitzel, St. Louis County. Mo. [21 1 AppL No. 73 Primary ExammerAndrew R. Juhasz 22] Filed Jan. 23, 1969 Assistant Examiner-Dav1d R. Melton [45] Patented Jan. 19, 1971 Aflorney-Koenig, Senniger, Powers and Leavitt [73] Assignee McNeilCorporation Akron, Ohio a corporation of Ohio TED Cl RS-TYPE ACK [54] AIR O-PERA J ABSTRACT: A scissors-type jack for a vehicle lift comprises a 18 Claims, 10 Drawing Figs. base, a scissors linkage above the base and a platform on the [)2] US. Cl 254/122, scissors linkage adapted to be raised to various elevations by 254/8 upward extension of the linkage from the base The linkage is l 3/22 adapted to be so extended by air cylinder actuated follower ollers which have a wedging action between the links of the 93, l04 122, H4, 126; 74/52l;92/l5, 19,24, 1 scissors linkage, the air cylinder being movable with thescis- 28 sors pivot. An auxiliary locking mechanism is provided for locking the follower rollers in various platform-elevating posi- [56] References Cned tions thereof to prevent collapse of the jack under load in the UNITED STATES PATENTS event of failure of the air cylinder to hold the follower rollers 2,862,689 12/1958 Dalrymple .v 254/l22X in platform-elevating position.

4 4 5 45 3 7 45 43 4/ r r i I r W q 1/ 7m ems mp] ltill !{1mmlllllllllllllllllllll W PATENTED JAN19I9YI sum 1 OF 4 AIR-OPERATED SCISSORS-TYPE JACK BACKGROUND OF THE INVENTION This invention relates to scissors jacks. and more particularly to an air-operated scissors jack useful. for example, as a platform lift for vehicles.

The invention relates especially to scissors jacks of the type such as shown in German Pat. Nos. 6() 4,l56 and l,l75,852 in which the links of the scissors mechanism of the jack are extended by the wedging action of follower rollers. As to the jacks shown in each of these German patents, the follower rollers are actuated by screw mechanism driven by an electric motor. It has been found to be desirable to provide for actuation of the follower rollers in a jack of this type by an air cylinder, rather than by mechanism driven by an electric motor, but this has involved problems as to the disposition of the air cylinder and as to safety in the event of loss of air pressure in the cylinder, and this invention solves these problems.

SUMMARY OF THE INVENTION Among the several objects of this invention may be noted the provision of a jack of the class described in which the fol lower rollers for actuating the scissors mechanism of the jack are actuated by a fluid cylinder, more particularly an air cylinder (as distinguished from being actuated by electric motor powered screw mechanism), and which is of simplified economical construction; and the provision of a jack such as described with a safety feature to prevent collapse of the jack in the event ofloss of pressure in the cylinder.

In general, a jack of this invention comprises a base, a scissors linkage extending upward from the base, a platform on the scissors linkage adapted to be raised to various elevations by upward extension of the linkage from the base. The scissors linkage comprises scissors links pivotally interconnected intermediate their ends by a scissors pivot which moves upward on extension of the linkage and downward on retraction of the linkage. Operation of the linkage is effected by the use of follower means between the links on one side of the pivot movable inward toward the pivot with a wedging action between the links for extending them to raise the platform to various elevations and movable outward away from the pivot for downward retraction of the linkage, and cylinder and piston means movable upward and downward with the scissors pivot and interconnected with the follower means for moving it inward to various platform-elevating positions. Additionally, an auxiliary means for locking the follower means against outward movement from any platform-elevating position thereof may be provided to prevent collapse of the jack under load in the event of failure of the cylinder and piston means to hold the follower means in platform-elevating position, said locking means being releasable to allow outward movement of the follower means for lowering the platform. Other objects and features will be in part apparent and in part pointed out hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective showing a jack constructed in accordance with this invention, the platform of the jack being shown in a raised position;

FIG. 2 is a vertical longitudinal section through the jack, also showing the platform in a raised position;

FIG. 3 is a left end view of the jack, showing the platform lowered;

FIG. 4 is a right end view of the jack, also showing the platform lowered;

FIG. 4A is an enlarged fragment of FIG. 4 with certain parts broken away and shown in section;

FIG. 5 is an enlarged vertical section on line 5-5 of FIG. 6;

FIG. 6 is a section similar to FIG. 2, with parts broken away, showing the scissors linkage of the jack in retracted position;

FIG. 7 is a section showing the auxiliary locking mechanism of the jack in a condition of partial raising of the platform;

FIG. 8 is an enlarged section on line 8-8 of FIG. 6; and

FIG. 9 is a section showing details of a valve for controlling the auxiliary locking mechanism.

Corresponding reference characters indicate corresponding parts throughout the several views of the drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawings, first more particularly to FIGS. I and 2. ajack constructed in accordance with this invention is shown to comprise a base 1, a scissors linkage 3 extending upward from the base, and a platform 5 on the scissors linkage adapted to be raised to various elevations by upward extension of the linkage from the base. The platform remains generally horizontal as it is raised and lowered. The linkage 3 comprises two pairs of scissors links, each pair being generally designated 7, the two links of each pair being designated 9 and 1 I. These links are pivotally interconnected intermediate their ends by a scissors pivot 13 which moves upward on extension of the linkage and downward on retraction of the linkage. At 15 is indicated follower means between the links on one side of the pivot (the left side, as illustrated) movable inward toward the pivot (i.e., toward the right) with a wedging action between the links for extending them to raise the platform to various elevations and movable outward away from the pivot (i.e., toward the left) for downward retraction of the linkage. At 17 is indicated cylinder and piston means movable upward and downward with the scissors pivot and interconnected with the follower means for moving it inward to various platformelevating positions. At 19 is generally indicated auxiliary means for locking the follower means against outward movement from any platform-elevating position thereof to prevent collapse of the jack under load in the event of failure of the cylinder and piston means to hold the follower means in platform-elevating position. This locking means is releasable, as will appear, to allow outward movement of the follower means for lowering the platform.

The jack is especially designed for use on a lift such as a four-post lift with the base 1 extending transversely between the runways of the lift on which a vehicle is parked, and movable longitudinally of the runways to various positions under the vehicle. For this purpose, the base 1 has angle brackets 21 at its ends carrying rollers 23 adapted to roll on tracks 25 extending inward from the runways 27 (see FIGS. 1 and 2). The base is constituted by a channel, the bottom of which is designated 29 and the side flanges of which are designated 31. The platform 5, which extends horizontally in the same plane as the base, comprises a channel 33, the bottom of which is designated 35 and the side flanges of which are designated 37. A pair of channel-section slides 39 are slidable endwise in the channel 33. Each of these slides carries three pivoted pads such as indicated at 41, 43 and 45. The slides are adapted to be moved to various positions relative to the channel 33, including positions extending from the ends of channel 33 as shown in FIG. 1, and the pads are adapted to be swung up to raised positions, as illustrated for pads 41 in FIG. 1 and for all the pads in FIGS. 3 and 4, for engagement by various under parts of a vehicle to be raised by the jack off the runways. Auxiliary pads such as indicated at 47 in FIG. 1 may be stored on brackets 49 mounted on a side flange 31 of the base 1. The channel 33 has side plates 51 extending down from its side flanges 37. These plates have inwardly directed flanges 53 at the top overlying outwardly directed tips 55 on the side flanges of the channel-section slides 39 for confining the latter in the channel 33.

The cylinder and piston means 17 comprises an air cylinder 57 having end heads 59 and 61. The cylinder also has side plates 63, which extend between the end heads 59 and 61 on opposite sides of the cylinder. Extending laterally outward from these cylinder side plates, generally at the center of length of the cylinder and in the central horizontal plane of the cylinder, are the scissors pivots 13 for the two pairs of scissors links. As shown in FIG. 4A, each of these pivots 13 is constituted by a pivot pin extending outward through a hole 65 in the side plate and having a head 67 received in an enlarged inner end portion 69 ofthe hole 65. The links 9 and 11 of each pair 7 of scissors links are pivoted generally centrally of their lengths on a respective pivot pin 13, and held thereon by a nut 71 threaded on a reduced-diameter outer end extension 73 of the pin. Link 9 is located on the inside of link 11. The links 9 and 11 have oppositely directed hubs as indicated at 75 on the pin 13 which provide for spacing of the links as appears best in FIG. 4A. The lower ends of links 9 of the two pairs of links are pivoted on the bottom 29 of the base 1 as indicated at 77. Extending transversely between the upper ends of links 9 is a roller assembly comprising a roller axle 79 having two rollers 81 thereon in rolling contact with the lower face of bottom 35 of the channel 33 of platform 5. The upper ends oflinks 11 are pinconnected at 83 to brackets 85 on the outside of side plates 51 of the platform, and extending transversely between the lower ends of links 11 is a roller assembly comprising a roller axle 87 having two rollers 89 thereon in rolling contact with the upper face of bottom 29 of the base 1. Additional rollers 91 are provided on the outside of links 11 at their lower ends. Lubrication fittings for the roller assemblies are indicated at 93.

The two pairs 7 of scissors links 9 and 11 are thus spaced from one another on opposite sides of the cylinder 57, and the cylinder 57, being interconnected via its side plates 63 with the scissors pivot pins 13, is adapted for conjoint movement with pins 13. The cylinder 57 has a piston 95 slidable therein. At 96 is generally indicated piston rod means movable with the piston. This comprises a first piston rod 97 extending toward the left as viewed in FIGS. 2 and 6 from the piston through an opening 99 in the cylinder end head 59. The follower means 15, which is mounted on the outer end of this piston rod, comprises a crossbar 101 of square cross section secured in position extending transversely of rod 97 at its outer end by means such as indicated at 103. Pins 105 extend V outward from the ends of bar 101. Each pin carries an inner follower roller 107 and an outer follower roller 109, held in place by a nut 111 threaded on a reduced-diameter end extension 113 of the pin. Lubrication fittings 115 are provided for lubricating the rollers. The inner rollers 107 are located for wedging action between links 9 (which are the inner links of the scissors-link pairs 7) and the outer rollers 109 are located for wedging action between the links 11 (which are the outer links of the scissors-link pairs) on the left side of the scissors pivot axis (i.e., the axis of pivot pins 13). The links 9 and 11 have opposed curved flanged cam edges as indicated at 117 and 119 on the left side of the pivot axis between which the follower rollers 107 and 109 are adapted to wedge on movement of the follower rollers in inward direction toward the scissors pivot axis (i.e., toward the right as viewed in FIGS. 2 and 6) to cause the links 9 and 11 to swing relative to one another on the scissors pivot axis for upward extension of the links for raising the platform 5. On movement of the follower rollers in outward direction away from the scissors pivot axis (i.e., movement toward the left as viewed in FIGS. 2 and 6), the links 9 and 11 are retracted downwardly (under the weight of the platform and the load supported thereby) for lowering of the platform. FIG. 6 shows the follower rollers in their extreme outward position and the links in their downward retracted position, and FIG. 2 shows the follower rollers in their extreme inward position and the links in their extreme extended position. It will be observed that as the links retract on outward movement of the follower rollers, the rollers 81 at the upper ends oflinks 9 and the rollers 89 and 91 at the lower ends of links 11 roll outward (i.e., toward the right as viewed in FIG. 2).

A conventional manually operated rotary air control valve 121 for controlling the operation of cylinder 57 is mounted on the outside of one of the flanges 31 of the base 1. One end of the head ofa tee fitting 123 is connected to the air inlet of this valve, and an air hose 125 extending from a compressed air supply is connected by a coupler 127 to the other end of the head of the tee fitting. An air line 129 is interconnected between valve 121 and the cylinder end head 59. The other cylinder end head 61 has a vent 131. Valve 121 has a handle 133 adapted to be turned from a vertical; shutoff position toward the right for supplying air via line 129 to the left end of the cylinder and toward the left'for venting air via line 129 from the left end of the cylinder. In'the vertical shutoffposition of the handle, the valve traps air in line 129 and in the cylinder on the left of piston 95. t

The piston rod means 96 further comprises a second piston rod 135 extending toward the right as viewed in FIGS. 2 and 6 from the piston through an opening 137 in the cylinder end head 61. Rod has an enlarged-diameter cylindric extension 139 threaded on its outer end providing an annular shoulder 141 facing toward the end head at the inner end of the extension. The locking means 19 comprises a plurality of individual locking members each designated 143 successively engageable with the rod 135 behind the shoulder 141 as the rod 135 moves toward the right as viewed in FIGS. 2 and 6 when the links 9 and 11 are extended to raise the platform 5 to lock the piston rod 135 and hence to lock the follower means 15 against outward movement in link-retracting direction until the locking members are retracted from their locking position behind the shoulder. Each locking member 143 is constituted by a rectangular plate having a notch 145 (see FIG. 5) extending upward from its bottom edge. The width of this notch is slightly greater than the diameter of the rod 135, and less than the diameter of the rod extension 139. The notch has a generally semicircular upper end adapted to seat on the rod 135. The locking plates 143 are arranged in a horizontal row above the axis of the rod 135 within a housing 147 extending toward the right from the right-hand cylinder end head 61. Each locking plate is disposed in a vertical plane at right angles to the piston rod axis, and successive plates are in face toface engagement, with each plate being individually vertically slidable between a raised retracted position relative to the rod 135 and a lowered locking position wherein its right-hand face is engageable by the shoulder 141.

The locking plate housing 147 has a horizontal top wall 149 and sidewalls 151, the latter being spaced a distance slightly greater that the width of a locking plate. The housing has an inner left-hand end wall 153 secured on the outside of the cylinder end head 61, and extends out toward the right from the cylinder 57, having an outer right-hand end wall 155. On the inside of the housing end walls 153 and 155 are guides 157 and 159 to the piston rod extension 139, with partitions 161 and 163 in the housing at the inner ends of these guides constituting end stops for the series of locking plates to hold the latter against sliding vendwise in the housing. With the piston 95 at the left end of its stroke, and the follower means 15 accordingly in its extreme outward position for downward retraction of links 9 and 11, the piston rod extension 139 extends through the locking plate housing 147 from one end thereof to the other, the ends of extension 139 being received in the guides 157 and 159 as shown in FIG. 6. Each and every one of the locking plates then occupies its raised retracted position with the corners of the lower edge of the locking plate on opposite sides of the notch 145 in the locking plate engaging the piston rod extension 139 (see the position of the locking plate 143 shown in solid lines in FIG. 5). As the piston 95 moves toward the right to draw the follower means 15 inward to raise the platform 5, the piston rod extension 139 moves toward the right and, as the shoulder 141 at its inner end passes each successive locking plate, the latter slides down by gravity in back of the shoulder 141 to its locking position in respect to the shoulder. See FIG. 7 showing the innermost or first locking plate 143 of the series down in its locking position behind the shoulder 141. It will be apparent therefrom that, as the piston rod extension 139 moves farther toward the right from its FIG. 7 position, the second locking plate will slide down behind the shoulder, then the third locking plate, and so on.

Means is provided for raising the locking plates 143 totheir retracted position clear of the shoulder 141 to enable the piston 95 to be moved toward the left for lowering the platform. As shown in F165. 2, 5 and 6, this means comprises an air cylinder 165 secured in vertical position between the sidewalls 151 of housing 147 at the bottom thereof generally centrally of the length of the housing. A piston 167 is slidable in cylinder 165 and extends up out of the upper end of the cylinder, which is open. A channel-section locking plate pusher 169 is mounted on the upper end of the piston 167, ex tending lengthwise of the housing 147 between the partitions 161 and 163. This pusher has side flanges 171 extending upward on opposite sides of the rod 135 and its extension 139, these flanges being engageable with the lower edges of the locking plates 143 for raising the latter on upward movement of the piston 167 and the pusher.

Operation of theair cylinder 165 is under control of a valve 173 mounted adjacent the main control valve 121 on the side flange 31 of the base 1. This valve 173 (see FIG. 9) is a pushbutton valve comprising a body 175 having a compressed air inlet 177 at its upper end which is connected to the lower end of the stem of the tee fitting 123. A tubular fitting 179 is threaded in a hole 181 extending upward from the lower end of the body and has a valve seat 183 at its upper end. This tubular fitting 179 is passaged as indicated at 185 for communication with a valve port 187. A valve member 189 is biased downward toward the valve seat 183 by a spring 191, and is adapted to be raised off the seat by a rod 193 loosely slidable in the fitting 179 having a push button 195 at its lower end. An air line 197 interconnects port 187 and the lower end of cylinder 165. The arrangement is such that on pushing up button 195, the valve member 189 is raised for delivery of air to the cylinder 165 to raise the piston 167 and locking plate pusher 169. On release of the button, valve member 189 closes, and air bleeds off from cylinder 165 through the space around the rod 193 (the loose fit of this rod in fitting 179 permitting this) for lowering of the piston 167.

Operation is as follows:

To raise the platform 5, the handle 133 of valve 121 is turned to the right to set valve 121 for supplying compressed air via line 129 to the left end of cylinder 57. Piston 95 and the piston rod means 96 accordingly move to the right, drawing the crossbar 101 and follower rollers 107 and 109 toward the right, and the follower rollers wedge apart the links 9 and 11 to extend the scissors linkage 3 and raise the platform 5. As the piston rod means 96 moves toward the right, locking plates 143 successively drop down onto the piston rod 135 behind the shoulder 141 at the inner end of the rod extension 139. The platform is raised to the desired elevation, and then handle 133 of valve 121 is turned back to its vertical position to shutoff the flow of air into cylinder 57 and to trap air in the cylinder.

The follower rollers 107 and 109 are maintained in their platform-elevating position (whatever it may be) by both the trapping of air in cylinder 57 on the left of piston 95 and the locking action of those locking plates 143 which have dropped down behind the shoulder 141. If, for any reason, such as rupture of line 129. the pressure of air trapped in the cylinder 57 should be lost, meaning that the cylinder and piston means 17 is no longer capable of holding these rollers 107 and 109 in their platform-elevating position, the rollers 107 and 109 remain locked against outward movement to prevent collapse of the jack under load due to the action of the locking plates 143.

To lower the platform 5, the handle 133 of valve 121 may be turned to the right to supply air to the cylinder 57 to move the piston rod means 96 to the right for the purpose of relieving pressure of shoulder 141 on the locking plates 143, and the button 195 of valve 173 may then be pushed upward to supply compressed air via line 197 to cylinder 165 to raise the piston 167 and the locking plate pusher 169. The side flanges 171 of the pusher 169 thereupon push up those locking plates 143 which have previously dropped into locking position to raise them clear of the shoulder 141. Handle 133 of valve 121 is turned to the left to vent the left end of cylinder 57 via line 129, and the follower rollers 107 and 109 thereupon move outward (i.e., toward the left, as illustrated) under the weight of the platform 5 and the load supported thereby for the lowering of the platform. The button 195 is held pushed upward until the platform is completely lowered, and then it is released to allow the piston 167 and locking plate pusher 169 to lower back downward to their retracted position. This occurs as a result of air bleeding off from cylinder via line 197 and the space around rod 193.

IN view of the above. it will be seen that the several objects of the invention are achieved and other advantageous results attained.

As various changes could be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

We claim: 7

1. A jack comprising a base, a scissors linkage extending upward from the base, a platform on the scissors linkage adapted to be raised to various elevations by upward extension of the linkage from the base, said linkage comprising scissors links pivotally interconnected intermediate their ends by a scissors pivot which moves upward on extension of the linkage and downward on retraction of the linkage, follower means between the links on one side of the pivot movable inward toward the pivot with a wedging action between the links for extending them to raise the platform to various elevations and movable outward away from the pivot for downward retraction of the linkage, and cylinder and piston means movable upward and downward with the scissors pivot and interconnected with the follower means for moving it inward to various platform-elevating positions.

2. A jack as set forth in claim 1 further having auxiliary means for locking the follower means against outward movement from any platform-elevating position thereof to prevent collapse of the jack under load in the event of failure of the cylinder and piston means to hold the follower means in platform-elevating position, said locking means being releasable to allow outward movement of the follower means for lowering the platform.

3. A jack as set forth in claim 2 wherein the cylinder and piston means comprises a cylinder movable with the pivot, a piston in the cylinder, and piston rod means movable with the piston and connected to the follower means, said locking means being interengageable with the piston rod means.

4. A jack as set forth in claim 2 wherein said locking means comprises a series of locking members successively movable from a retracted position to a locking position engageable by the piston rod means to lock the latter against movement in linkage-retracting direction.

5. A jack as set forth in claim 4 further comprising cylinder and piston means for moving said locking members to their retracted position.

6. A jack as set forth in claim 4 wherein said piston rod means comprises a first rod extending through one end of the cylinder to a connection with the follower means and a second rod extending through the other end of the cylinder having a shoulder facing said other end of the piston, and having means carrying said locking members at said other end of the cylinder, said locking members being successively movable behind said shoulder as the piston rod means moves in lint. age-extending direction.

7. A jack as set forth in claim 6 wherein said second rod has an outer end extension of larger diameter than the second rod, each of said locking members comprising a plate located above the second rod disposed in a plane at right angles to the axis of the rod, said plates being arranged in face-to-face engagement, each plate being individually vertically slidable between a raised retracted position wherein its lower end en gages said extension and a lowered locking position wherein one face thereof is engageable by the inner end of said exten sion acting as said shoulder.

8. A jack as set forth in claim 7 wherein each plate has a notch extending upward from its bottom edge adapted to receive said second rod.

9. A jack as set forth in claim 8 further comprising cylinder and piston means for raising said plates.

10. 'A jaek'as set forth in claim 9 wherein said cylinder and- .piston means comprises a cylinder having scissors pivots extending laterally outward from opposite sides thereof, and said scissors linkage comprises two pairs of scissors links. one pair being pivoted on the scissors pivot on one side of the cylinder. and the other pair being pivoted on the scissors pivot on the other side of the cylinder.

12.Ajack as set forth in claim 11 wherein said cylinder has a piston therein and a rod extending from the piston through one end of the cylinder, said follower means comprising a crossbar on the outer end of said rod and follower rollers on the crossbar between the scissors links.

13. A jack as set forth in claim 12 having auxiliary means mounted on the outside of the other end of the cylinder for locking the piston and follower rollers against outward move- 1 ment from any platformelevating position thereof to prevent cylinder having an outer end extension of larger diameter than said second rod providing a shoulder at the inner end of said extension facing said other end of the cylinder. said locking means comprising a housing on said other end of the cylinder, a plurality of locking plates arranged in a'row above the piston rod axis in said housing with each plate in a plane at right angles to said axis, said plates being arranged in face-.to-face engagement, each plate being individually vertically slidable between a raised retracted position wherein its lower end engages said extension and a lowered locking position wherein one face thereofis engageable by said shoulderi said extension extending through said 'housing and holding all said plates raised when the platform is fully-lowered.

15. A jack as set forth in claim 14 wherein each plate has a notch extending upward from its bottom edge adapted to receive said second piston rod, the width of said notch being less than the diameter of said extension.

16. A jack as set forth in claim 15 further comprising cylinder and piston means carried by said housing for raising said plates.

17. Ajack as set forth in claim 16 wherein said cylinder and piston means for raising 'said plates comprises a cylinder mounted below said plates, a piston extending up out of said cylinder, and a pusher on the upper end of said piston having side members extending upward on opposite sides of said second rod and said extension engageable with the lower edges of said plates.

18. A jack as set forth in claim 17 having a valve for controlling the supply and exhaust of air from the cylinder for the follower rollers and adapted to trap air in this cylinder, and a valve operable to supply air to the cylinder for raising the locking plates and adapted on release to bleed air from the latter cylinder. 

1. A jack comprising a base, a scissors linkage extending upward from the base, a platform on the scissors linkage adapted to be raised to various elevations by upward extension of the linkage from the base, said linkage comprising scissors links pivotally interconnected intermediate their ends by a scissors pivot which moves upward on extension of the linkage and downward on retraction of the linkage, folloWer means between the links on one side of the pivot movable inward toward the pivot with a wedging action between the links for extending them to raise the platform to various elevations and movable outward away from the pivot for downward retraction of the linkage, and cylinder and piston means movable upward and downward with the scissors pivot and interconnected with the follower means for moving it inward to various platform-elevating positions.
 2. A jack as set forth in claim 1 further having auxiliary means for locking the follower means against outward movement from any platform-elevating position thereof to prevent collapse of the jack under load in the event of failure of the cylinder and piston means to hold the follower means in platform-elevating position, said locking means being releasable to allow outward movement of the follower means for lowering the platform.
 3. A jack as set forth in claim 2 wherein the cylinder and piston means comprises a cylinder movable with the pivot, a piston in the cylinder, and piston rod means movable with the piston and connected to the follower means, said locking means being interengageable with the piston rod means.
 4. A jack as set forth in claim 2 wherein said locking means comprises a series of locking members successively movable from a retracted position to a locking position engageable by the piston rod means to lock the latter against movement in linkage-retracting direction.
 5. A jack as set forth in claim 4 further comprising cylinder and piston means for moving said locking members to their retracted position.
 6. A jack as set forth in claim 4 wherein said piston rod means comprises a first rod extending through one end of the cylinder to a connection with the follower means and a second rod extending through the other end of the cylinder having a shoulder facing said other end of the piston, and having means carrying said locking members at said other end of the cylinder, said locking members being successively movable behind said shoulder as the piston rod means moves in linkage-extending direction.
 7. A jack as set forth in claim 6 wherein said second rod has an outer end extension of larger diameter than the second rod, each of said locking members comprising a plate located above the second rod disposed in a plane at right angles to the axis of the rod, said plates being arranged in face-to-face engagement, each plate being individually vertically slidable between a raised retracted position wherein its lower end engages said extension and a lowered locking position wherein one face thereof is engageable by the inner end of said extension acting as said shoulder.
 8. A jack as set forth in claim 7 wherein each plate has a notch extending upward from its bottom edge adapted to receive said second rod.
 9. A jack as set forth in claim 8 further comprising cylinder and piston means for raising said plates.
 10. A jack as set forth in claim 9 wherein said cylinder and piston means for raising said plates comprises a cylinder mounted below said plates, a piston extending up out of said cylinder, and a pusher on the upper end of said piston having side members extending upward on opposite sides of said second rod and said extension engageable with the lower edges of said plates.
 11. A jack as set forth in claim 1 wherein said cylinder and piston means comprises a cylinder having scissors pivots extending laterally outward from opposite sides thereof, and said scissors linkage comprises two pairs of scissors links, one pair being pivoted on the scissors pivot on one side of the cylinder, and the other pair being pivoted on the scissors pivot on the other side of the cylinder.
 12. A jack as set forth in claim 11 wherein said cylinder has a piston therein and a rod extending from the piston through one end of the cylinder, said follower means comprising a crossbar on the outer end of said rod and follower rollers on the crossbar between the scissors links.
 13. A jack as set forth in claiM 12 having auxiliary means mounted on the outside of the other end of the cylinder for locking the piston and follower rollers against outward movement from any platform-elevating position thereof to prevent collapse of the jack under load in the event of failure of the cylinder and piston means to hold the follower rollers in platform-elevating position, said locking means being releasable to allow outward movement of the follower rollers for lowering the platform.
 14. A jack as set forth in claim 13 wherein said piston has a second piston rod extending through said other end of the cylinder having an outer end extension of larger diameter than said second rod providing a shoulder at the inner end of said extension facing said other end of the cylinder, said locking means comprising a housing on said other end of the cylinder, a plurality of locking plates arranged in a row above the piston rod axis in said housing with each plate in a plane at right angles to said axis, said plates being arranged in face-to-face engagement, each plate being individually vertically slidable between a raised retracted position wherein its lower end engages said extension and a lowered locking position wherein one face thereof is engageable by said shoulder, said extension extending through said housing and holding all said plates raised when the platform is fully lowered.
 15. A jack as set forth in claim 14 wherein each plate has a notch extending upward from its bottom edge adapted to receive said second piston rod, the width of said notch being less than the diameter of said extension.
 16. A jack as set forth in claim 15 further comprising cylinder and piston means carried by said housing for raising said plates.
 17. A jack as set forth in claim 16 wherein said cylinder and piston means for raising said plates comprises a cylinder mounted below said plates, a piston extending up out of said cylinder, and a pusher on the upper end of said piston having side members extending upward on opposite sides of said second rod and said extension engageable with the lower edges of said plates.
 18. A jack as set forth in claim 17 having a valve for controlling the supply and exhaust of air from the cylinder for the follower rollers and adapted to trap air in this cylinder, and a valve operable to supply air to the cylinder for raising the locking plates and adapted on release to bleed air from the latter cylinder. 